JP2016519706A - Method for adjusting PLA bead foam - Google Patents

Abstract

The present invention relates to a method for preparing PLA beads, and more particularly to a foamed PLA bead foam. Furthermore, the present invention relates to a method for preparing a molding by sintering PLA beads. The method includes A) supplying unfoamed PLA pellets, B) heating the unfoamed PLA pellets to an annealing temperature and saturating with a blowing agent, and C) maintaining the PLA pellets at the annealing temperature; Saturating with a blowing agent; and D) reducing the saturated PLA pellets of step C) to room temperature and producing foamed PLA bead foam. [Selection] Figure 1

Description

  The present invention relates to a method for preparing PLA beads, and in particular, relates to a foamed PLA bead foam. Furthermore, the present invention relates to a method for preparing a molding by sintering PLA beads.

  Foamed polymer bead foam is a non-degradable polymer that is widely used for packaging, thermal insulation and soundproofing applications, and in more advanced fields such as automobile manufacturing. In all cases, the physical and mechanical properties of the bead foam are very important to obtain a high quality end product. In addition, since the boundary of the beads becomes a crack generation point, it is very important to achieve good adhesion between the beads and sufficient sintering.

  Expanded polystyrene (EPS), expanded polyethylene (EPE), and expanded polypropylene (EPP) are well-known bead foams that are utilized on a large scale in the current market. They are available for various applications based on their mechanical and physical properties. Polylactic acid (PLA) is one of increasing importance because of its renewable resources, biocompatibility, and biodegradability, sufficient mechanical and thermal properties. Foamed PLA (EPLA) can be a very good replacement for EPS in everyday applications, but one of the most challenging problems recently is PLA crystallization. PLA crystallization occurs very slowly and can have a significant impact on the production of PLA bead foams. Therefore, in order to achieve high quality beads with good cohesion, it is very important to improve PLA crystallization and thereby improve foaming properties.

  PLA is a thermoplastic aliphatic polyester polymer derived from renewable resources such as corn starch and sugar cane. PLA can be decomposed even in an environment where petroleum-derived polymers do not decompose. In recent years, PLA has attracted attention as an alternative to polyester (PS) products. Currently, foamed PS (EPS) bead foam is widely used in various applications such as packaging, thermal insulation and sound insulation, construction, and cushioning materials. Since EPS is not biodegradable and is not environmentally friendly, PLA bead foam can be a suitable replacement for these EPS products.

  Currently, few companies produce foamed PLA (EPLA) bead foam, but good sintering between beads is still crucial to produce a three-dimensional final foam product with strong mechanical properties. It is a request.

  The object of the present invention is to achieve the desired three-dimensional final bead foam by creating two crystal melting peaks during the foaming process.

This method is a method of adjusting foamed PLA bead foam,
A) supplying unfoamed PLA pellets;
B) heating the unfoamed PLA pellets to an annealing temperature and saturating with a foaming agent;
C) maintaining the PLA pellet at the annealing temperature and saturating with the blowing agent;
D) decompressing the saturated PLA pellet of step C) and cooling to room temperature to produce expanded PLA bead foam;
Is provided.

We believe that the first crystal melting peak is the result of incomplete crystals that melt at lower temperatures and serves to melt and bond individual beads in the bead structure being processed. However, the second peak protects the foamed bead shape. The newly generated crystal peak during the isothermal gas impregnation, i.e., saturating with blowing agent, results from the completeness of the crystal phase from the unmelted crystal, which has higher orientation and higher melting temperature than the original peak. Have. The melting temperature of this peak is typically higher than the annealing temperature. The inventors believe that the fewer the number of complete crystals that melt when the temperature is raised to the annealing point, the more complete crystals exist without melting even above the annealing temperature. During isothermal processing, the T _mhigh due to _unmelted crystals is higher due to higher integrity and thicker layers, while more crystals are attributed to the higher melting peak, so the first one that forms during the cooling process. The peak portion becomes smaller.

  The annealing temperature is preferably in the range of 60 ° C to 180 ° C.

  The annealing time in step C) is preferably in the range of 10 minutes to 300 minutes.

  In step A), the PLA pellets are preferably selected from the group of linear PLA, branched PLA, and PLA copolymers having various contents of D-lactide, or combinations thereof.

  In step A), the PLA pellets preferably contain one or more additives selected from the group of chain extenders, nanoparticles, fine solid particles, fibers, lubricants.

  In step A), the PLA pellets are preferably mixed with polyolefin and / or polyester.

  The present invention further relates to a foamed PLA bead foam having two crystal melting peaks.

  In a preferred embodiment, the interval between the two peaks is in the range of 5 ° C to 25 ° C, and preferably the interval between the two peaks is in the range of 10 ° C to 20 ° C.

  The PLA beads are preferably selected from the group of linear PLA, branched PLA, and PLA copolymers having various contents of D-lactide, or combinations thereof. In a preferred embodiment, the PLA beads comprise one or more additives selected from the group of chain extenders, nanoparticles, microsolid particles, fibers, lubricants. The PLA beads preferably comprise a polyolefin and / or polyester.

  The invention further relates to a method for the molding of expanded beads. In this method, the expanded beads are sintered in the presence of hot air and / or steam, and the above PLA beads are used as the expanded beads.

  The present invention further relates to a foamed molding based on PLA beads having two crystal melting peaks which are sintered.

  Furthermore, the invention relates to the use of foamed moldings for packaging, insulation and sound insulation, construction and cushioning.

FIG. 1 is a diagram showing a DSC heating curve of a PLA sample at a heating rate of 10 ° C. per minute. FIG. 2 shows a study of the two peaks of a branched-chain PLA sample annealed at various temperatures for 60 minutes. FIG. 3 shows a study of the two peaks of a linear PLA sample annealed at various temperatures for 60 minutes. FIG. 4 shows a study of the two peaks of a branched chain PLA sample annealed at 148.5 ° C. for various annealing times. FIG. 5 is a diagram showing a comparison of melting peaks on the low temperature side and high temperature side of branched-chain PLA at different annealing times and the final crystallinity of the annealed sample. FIG. 6 shows a study of the two peaks of a branched PLA sample annealed at 4 temperatures for 60 minutes. FIG. 7 shows a study of the two peaks of a branched PLA sample annealed at four temperatures for 300 minutes. FIG. 8 shows a comparison between two peaks of a branched PLA sample when annealed for 60 minutes and 180 minutes. FIG. 9 shows expanded PLA beads that are saturated for 15-60 minutes at a saturation temperature between 120 ° C. and 124 ° C. and a CO ₂ pressure of 800 psi. FIG. 10 is a graph showing the melting temperature and crystallinity of branched-chain PLA annealed at different annealing temperatures. FIG. 11 is a diagram showing the melting temperature and crystallinity of linear PLA annealed at various temperatures by DSC.

  In order to achieve the desired two-peak structure, improved crystallization of PLA is required. This modification of crystallization will provide not only the possibility of generating two peaks suitable for bead sintering, but also an improvement in the inherently poor foaming properties of PLA. In particular, the crystals formed during the saturation step can dramatically affect the cell nucleation and foaming properties of PLA bead foam, which is preferred to achieve very high quality PLA bead foam. it can. Essentially low melting strength PLA molecules become high melting strength materials. This is because PLA molecules bind through the crystal and improve the low melting strength of PLA. As a result, PLA foaming ability is improved by minimizing both foam loss and cell agglomeration. A crystallinity that is too high will reduce the foam's foaming capacity due to increased strength, so the degree of crystallinity caused during the saturation of the blowing agent will reduce the appropriate saturation temperature and time. It needs to be controlled by selecting.

  A standard differential scanning calorimetry (DSC) study was performed to investigate the evolution of the thermal behavior of polylactic acid (PLA) for the purpose of producing expanded PLA bead foam (EPLA) by a batch process. For linear and branched PLA with two different crystallization rates, the effect of various annealing temperatures and times on the generation of two crystal melting peaks is investigated.

[Experimental procedure]
Materials: In this experiment, one commercially available linear PLA and one long chain branched (LCB) PLA were used. Linear PLA was modified with 0.5 wt% talc. Branched PLA was prepared by melt extruding linear PLA with 0.7 wt% epoxy-based polyfunctional chain extender and 0.5 wt% talc.

Differential Scanning Calorimetry (DSC): The effect of annealing temperature (T _s ) and time (t _s ) on the generation and evolution of two crystal melting peaks for linear and branched PLA samples was measured by DSC 2000 (TA Instruments). Was investigated with a DSC apparatus. In the absence of blowing agent, the samples were heated from room temperature to different annealing temperatures at a heating rate of 30 ° C. per minute to simulate a batch process for two crystal melting peaks. Subsequently, the sample was annealed for 60 minutes in order to investigate the effect of annealing temperature on the feasibility of generating two crystal melting peaks in the sample. After annealing at T _s , the sample was cooled from T _s to room temperature at a rate of 20 ° C. per minute. In order to investigate the effect of annealing on the two peak generations, a second heating run was performed by heating the sample to 200 ° C. at a heating rate of 10 ° C. per minute.

[Figure 1]
FIG. 1 shows a heating thermograph of linear and branched PLA samples before the annealing step for simulation purposes. As shown, the melting temperature of both samples is about 148 ° C. Linear PLA also exhibits a higher cold crystallization peak temperature (T _cc ) than branched PLA due to slower crystallization rates, while cold crystals can be formed much faster in branched PLA. This indicates that the branched structure has a faster crystallization rate because the branched structure acts as a crystal nucleus.

  An isothermal annealing step was applied to investigate the formation of two melting peaks, both linear and branched PLA.

FIG. 2 shows a heating graph of the branched PLA after isothermal annealing at various temperatures for 60 minutes. As shown, most of the crystals present in the PLA sample remained unmelted during the isothermal annealing at 140 ° C. because the annealing temperature was much lower than Tm. However, when annealing at high temperatures, the crystal layer becomes thicker and crystal rearrangement occurs, so the main melting peak shifts to higher temperatures due to crystal integrity. Furthermore, by increasing the annealing temperature to 145 ° C., during the annealing process, some parts of the crystal melt and parts of the crystal that do not melt begin to rearrange and appear as the second melting peak in FIG. A thick layer is formed. The first peak appears as a result of the cooling step after annealing. These are crystals that melt during annealing and can be formed as the original melting peak (first peak) during cooling. Furthermore, increasing the annealing temperature to 150 ° C. increases the amount of crystals that melt during the annealing process, thus reducing the amount of crystals that remain unmelted during annealing. Thus, during cooling after annealing, a large portion of the crystal forms the original first peak and the second peak, the thicker layer that has been completed, becomes smaller. In fact, as the saturation temperature is increased, the amount of crystals that do not melt decreases, so the second peak is not dominant. As a result, the first peak portion formed during the cooling process increases. When the annealing temperature is increased to 152.5 ° C., all the existing crystals melt during the annealing process, so after the cooling process, all the crystals form only the original first peak.
[Figure 2]

FIG. 10 shows the melting temperature and crystallinity results of the graph shown in FIG. This result shows that when a branched-chain PLA sample is annealed near its melting peak (148 ° C.), two crystalline melting peaks are generated in the branched-chain PLA sample having a maximum total crystallinity of about 30%. Indicate.
[FIG. 10]

[Fig. 3]
FIG. 3 shows a heating graph of linear PLA after annealing isothermally at various temperatures for 60 minutes. As shown, even a linear PLA with lower crystallinity can produce two peaks with a reasonably high total crystallinity (FIG. 11) when annealed at about 143 ° C. The second peak clearly appears at about 143 ° C. as well as the branched PLA, probably because of the higher regularity of the chain present in the linear PLA.

According to FIGS. 10 and 11, the spacing between the two peaks produced is about 15 ° C. This can be regarded as a processing condition width (window) in the steam chamber molded product manufacturing step.
[Fig. 11]

[FIGS. 4 and 5]
FIG. 4 shows the results of annealing a branched chain PLA sample at 148.5 ° C. for various annealing times. When the temperature is increased to T _s and immediately cooled (0 minutes), more complete, unmelted crystals appear as a second peak, and more molten crystals are removed during the cooling process. Re-forms and appears as the first peak. By increasing the annealing time from 10 minutes to 300 minutes, the unmelted crystals forming the second peak will experience longer diffusion times to form thicker layers and more complete crystals, so during the cooling process Fewer crystals can be reshaped as the first peak. In fact, increasing the annealing time results in the formation of more complete crystals, increasing the amount of the second peak with higher integrity, and leaving few crystals that become the first peak during the cooling process. In other words, during the longer annealing time, the crystal integrity was further increased and the second high temperature melting peak (T _mhigh ) was formed at a higher temperature. Furthermore, the total crystallinity of the annealed samples was also improved after longer annealing times. On the other hand, it is an important problem in steam chamber molding, and the interval between two peaks indicating the processing condition width is shown in FIG. FIG. 5 shows that the spacing between the peaks becomes larger during the longer annealing time. Since the second peak is formed on the higher temperature side due to the longer annealing time, the distance from the first peak is further increased.

[FIGS. 6 and 7]
6 and 7 show the results of annealing a branched-chain PLA sample at four annealing temperatures for 60 minutes and 180 minutes, respectively. In the case where the annealing temperature is 150 ° C. and the annealing time is 60 minutes, a large part of the crystal melts during the annealing process and re-forms during the cooling process. Occupies a very small part. However, if the annealing time is 180 minutes, the amount of complete crystals increases, the second peak becomes larger, and the microcrystals are attributed more evenly to the first and second peaks.

  Similar behavior appears for other annealing temperatures. For example, at 147 ° C., after an annealing time of 60 minutes, the second peak forms a larger capacity than the first peak, but the longer annealing time increases the amount of the second crystal melting peak. To do. When the annealing temperature is increased to 152.5 ° C., all crystals still melt during annealing after 180 minutes and re-form as the first peak during the cooling process.

[Fig. 8]
FIG. 8 shows the spacing between the two peaks of the samples annealed at three annealing temperatures for 60 minutes and 180 minutes. In all three selected annealing temperatures, the spacing between the peaks is approximately 2-3 ° C. higher when the annealing time is 180 minutes.

  We investigated the possibility of generating two crystal melting peaks for PLA to produce expanded PLA bead foam (EPLA). Both linear and branched PLA with different crystallinity can produce two crystal melting peaks with a reasonably high final crystallinity. The second peak appears due to crystal integrity, but the first peak forms during the cooling step after annealing. On the other hand, longer annealing times not only increase the amount of crystals that are perfected, but also increase the melting peak temperature due to higher crystal integrity.

  In addition, polylactic acid (PLA) bead foam was manufactured using a one-step manufacturing method that produced two crystalline melting peak structures. The two crystal melting peaks of PLA are used for self-diffusion sintering between bead foams during the molding stage. Sintered beads are used to produce three-dimensional foam products with excellent mechanical properties by partial melting of crystals.

  In the sintering technique, the high temperature melting peak crystals formed during the isothermal saturation step in the batch based bead foaming process are used to maintain the bead geometry even at the high temperatures required for commercial products. The formation of this crystal melting peak at a higher temperature than the original crystal is due to the crystal integrity induced during the blowing agent saturation stage, which is carried out near the melting temperature. After saturation with the blowing agent, during cooling, foaming occurs and at the same time a lower temperature melting peak is formed.

  In the vapor chamber molding process (ie, in the final processing step to cause sintering of the beads and produce a three-dimensional foam product), foamed PLA beads are fed into the mold cavity. Subsequently, hot steam, which is the temperature between the cold melting peak and the newly generated hot melting peak, is fed into the mold cavity to heat the beads. When exposed to this hot vapor, the cold melting peak crystals of the beads melt. This causes the beads to sinter together, but the unmelted hot melt peak crystals contained in the beads maintain the overall bead geometry. Low temperature melting peak crystals melt and molecular diffusion occurs between the beads. As a result, the overall geometric shape of the beads is maintained, and the foamed product is in the shape of a mold cavity and has a good geometric shape due to excellent bead sintering formed from the diffusion of molecules between the beads. Has a shape. This high quality sintering of PLA will give the product excellent mechanical properties.

  In the present invention, unfoamed PLA pellets were saturated at elevated temperature while impregnating the pellets with a physical blowing agent in a laboratory scale autoclave system. The range of saturation temperature to produce the two crystal melting peak structures was determined by the PLA melting temperature and the amount of blowing agent dissolved in the PLA pellets. The degree of plasticization of the blowing agent dissolved in the PLA pellets changed the saturation temperature range required to produce two crystal melting peaks in the PLA expanded beads. The PLA saturation process was performed at various times when a sufficient amount of high melting temperature complete crystals could be formed.

The saturation temperature range used was between 60 ° C. and 180 ° C. while varying the amount of impregnation of the used blowing agent. The impregnation pressure supplied in the chamber was 70 psi or more. For the purposes of the present invention, PLA polymers are copolymers with various contents of D-lactide. Different degrees of branching PLA polymers were used, including various types of chain extenders. In addition, PLA polymers mixed with different particles including nanoparticles, micronized additives, fibers, and lubricants were also used. Another option for bead foam with two crystal melting peak structures is a PLA blend with polyolefin and polyester. Different types of blowing agents are organic compounds such as hydrocarbons such as butane and pentane. Then, there is a volatile blowing agent such as CO _2. Such a mixture of different blowing agents is also another option.

  A suspension medium such as water was used to suspend the PLA pellets during saturation. The suspending medium may be mixed with various suspension stabilizers such as organic or inorganic compounds that do not dissolve in the suspending medium. Various surfactants, such as silicone oil, may be applied to the suspending medium as a suspending aid.

  The crystallization rate (ie nucleation and expansion) of the PLA sample can be influenced by the saturation temperature, the amount of blowing agent dissolved, and the saturation time. Thus, the high melting temperature perfect crystals produced can have different crystallization rates during the saturation step. Thus, the cell structure and foaming of PLA bead foam can be influenced by changing the crystallization rate under different saturation conditions. Changes in the number of nucleated crystals and their size can affect the nucleation of cells of different components. Changes in the crystallization rate can also affect the melting strength of PLA. The expansion ratio of expanded PLA beads will be affected by changes in melt strength caused by different crystallization rates.

PLA bead foam with two crystalline melting peak structures can be stored at room temperature. The expansion ratio of the PLA bead foam was 100 times or less, and the closed cells were dominant. The cell density of the bead foam ranged between 10 ⁶ to 10 ¹³ cells / cm ³ .

  The foamed beads can be sintered during a foam molding process using steam or hot air to produce three-dimensional foam products for various applications such as packaging, insulation, and construction. Sintered beads have excellent mechanical properties because low melting temperature crystals (i.e., first peak crystals) melt during foam molding and molecular diffusion occurs between the beads. This treatment contributed to the strong self-sintering action of the foamed beads. On the other hand, the full crystals of high melting temperature produced strongly contributed to the maintenance of foamed bead morphology and structure.

A branched PLA pellet was used as an example. PLA has a D-lactide content of 4.6%. CO ₂ having a purity of 99.98% by Linde Gas was supplied as a saturated gas.

Branched-chain PLA samples were saturated in a laboratory scale autoclave system at a temperature range between 115 ° C. and 130 ° C. with a CO ₂ pressure of 700 psi to 900 psi. The saturation time was varied between 15 and 60 minutes.

First, PLA pellets were heated to different saturation temperatures and impregnated with dissolved gas for various saturation times. After saturation, the PLA sample was discharged with the suspension medium, stabilizer, surfactant, and saturated CO ₂ gas by opening the ball valve provided at the bottom of the chamber. PLA foam beads were formed by rapidly depressurizing and cooling. FIG. 9 shows the cell structure of a foamed sample saturated at a temperature of 120 ° C. to 124 ° C. CO ₂ pressure was 800 psi, saturation time was between 15 minutes to 60 minutes. Expanded beads with two crystal melting peak structures had an expansion ratio between 5 and 40 times.
[Fig. 9]

Claims (15)

A) supplying unfoamed PLA pellets;
B) heating the unfoamed PLA pellet to an annealing temperature and saturating with a foaming agent;
C) maintaining the PLA pellet at the annealing temperature and saturating with the blowing agent;
D) decompressing the saturated PLA pellet of step C) and cooling to room temperature to produce expanded PLA bead foam;
A method for preparing a foamed PLA bead foam, comprising:   The method of claim 1, wherein the annealing temperature is in the range of 60C to 180C.   The method according to claim 1 or 2, wherein the annealing time in step C) ranges from 10 minutes to 300 minutes.   4. In step A), the PLA pellets are selected from the group of linear PLA, branched PLA, and PLA copolymers having various contents of D-lactide, or combinations thereof. the method of.   5. The method of claim 4, wherein in step A) the PLA pellet comprises one or more additives selected from the group of chain extenders, nanoparticles, fine solid particles, fibers, lubricants.   6. The method according to claim 4 or 5, wherein in step A) the PLA pellets are mixed with a polyolefin and / or polyester.   Expanded PLA bead foam with two crystal melting peaks.   The PLA bead of claim 7 wherein the spacing between the two peaks ranges from 5 ° C to 25 ° C.   The PLA bead of claim 8 wherein the spacing between the two peaks is in the range of 10 ° C to 20 ° C.   The PLA bead of any of claims 7 to 9, wherein the PLA bead is selected from the group of linear PLA, branched PLA, and PLA copolymers having various contents of D-lactide, or combinations thereof.   The PLA bead according to any one of claims 7 to 10, wherein the PLA bead includes one or more additives selected from the group of chain extenders, nanoparticles, fine solid particles, fibers, and lubricants.   The PLA bead according to claim 7, wherein the PLA bead includes polyolefin and / or polyester. A method for molding foam beads,
The foamed beads are sintered in the presence of hot air and / or steam;
A method for forming foam beads, wherein the PLA beads according to claim 7 are used as the foam beads.   Foam molding based on PLA beads with two crystal melting peaks sintered.   Use of the foamed molding according to claim 14 for packaging, insulation and sound insulation, construction and cushioning.